The present invention relates to the use of latent acid catalysts in coating compositions.
Coating compositions which are capable of acid catalyzed crosslinking generally contain a catalytic amount of an acid catalyst. The acid catalyst is added in order to accelerate crosslinking and thereby reduce the overall time required for cure. When an acid catalyst is utilized, it is advantageous to have the catalyst present as the free acid since in this manner a rapid cure can be efficiently achieved. However, the presence of the free acid may also cause problems in storage stability; that is, the coating composition will exhibit a tendency to gel and harden during the storage term, thus becoming unfit for use.
As a means of circumventing these difficulties, latent or blocked acid catalysts are often utilized to delay the action of crosslinking agents and otherwise postpone the curing mechanism.
Latent acid catalysts are formed by preparing a derivative of an acid catalyst such as para-toluenesulfonic acid (pTSA) or other sulfonic acids. For example, a well-known group of blocked acid catalysts are amine salts of aromatic sulfonic acids, such as pyridinium para-toluenesulfonate. Such sulfonate salts are less active than the free acid in promoting crosslinking. During cure, the catalysts are activatived by heating.